1. Field of the Invention
This invention relates generally to integrated circuit fabrication and, more particularly, to masking techniques.
2. Description of the Related Art
As a consequence of many factors, including demand for increased portability, computing power, memory capacity and energy efficiency, integrated circuits are continuously being reduced in size. The sizes of the constituent features that form the integrated circuits, e.g., electrical devices and interconnect lines, are also constantly being decreased to facilitate this size reduction.
The trend of decreasing feature size is evident, for example, in memory circuits or devices such as dynamic random access memories (DRAMs), flash memory, static random access memories (SRAMs), ferroelectric (FE) memories, etc. To take one example, DRAM typically comprises millions of identical circuit elements, known as memory cells. A DRAM memory cell typically consists of two electrical devices: a storage capacitor and an access field effect transistor. Each memory cell is an addressable location that can store one bit (binary digit) of data. A bit can be written to a cell through the transistor and can be read by sensing charge in the capacitor. By decreasing the sizes of the electrical devices that constitute a memory cell and the sizes of the conducting lines that access the memory cells, the memory devices can be made smaller. Additionally, storage capacities can be increased by fitting more memory cells on a given area in the memory devices. Other memory designs can integrate access and storage devices or omit access devices (e.g., cross-point MRAM, PCRAM, etc.).
The continual reduction in feature sizes places ever greater demands on the techniques used to form the features. For example, photolithography is commonly used to pattern features, such as conductive lines. The concept of pitch can be used to describe the sizes of these features. Pitch is defined as the distance between an identical point in two neighboring features of a repeating pattern. These features are typically defined by spaces between adjacent features, which spaces are typically filled by a material, such as an insulator. As a result, pitch can be viewed as the sum of the width of a feature and of the width of the space on one side of the feature separating that feature from a neighboring feature. However, due to factors such as optics and light or radiation wavelength, photolithography techniques each have a minimum pitch below which a particular photolithographic technique cannot reliably form features. Thus, the minimum pitch of a photolithographic technique is an obstacle to continued feature size reduction.
“Pitch doubling” or “pitch multiplication” is one proposed method for extending the capabilities of photolithographic techniques beyond their minimum pitch. A pitch multiplication method is illustrated in FIGS. 1A-1F and described in U.S. Pat. No. 5,328,810, issued to Lowrey et al., the entire disclosure of which is incorporated herein by reference. With reference to FIG. 1A, a pattern of lines 10 is photolithographically formed in a photoresist layer, which overlies a layer 20 of an expendable material, which in turn overlies a substrate 30. As shown in FIG. 1B, the pattern is then transferred using an etch (preferably an anisotropic etch) to the layer 20, thereby forming placeholders, or mandrels, 40. The photoresist lines 10 can be stripped and the mandrels 40 can be isotropically etched to increase the distance between neighboring mandrels 40, as shown in FIG. 1C. A layer 50 of spacer material is subsequently deposited over the mandrels 40, as shown in FIG. 1D. Spacers 60 (FIG. 1E), i.e., the material extending or originally formed extending from sidewalls of another material, are then formed on the sides of the mandrels 40. The spacer formation is accomplished by preferentially etching the spacer material from the horizontal surfaces 70 and 80 in a directional spacer etch, as shown in FIG. 1E. The remaining mandrels 40 are then removed, leaving behind only the spacers 60, which together act as a mask for patterning, as shown in FIG. 1F. Thus, where a given pitch previously included a pattern defining one mask element and one space, the same width now includes two mask elements and two spaces, with the spaces defined by, e.g., the spacers 60. As a result, the smallest feature size that can be formed with a photolithographic technique is effectively decreased.
While the pitch is actually halved in the example above, this reduction in pitch is conventionally referred to as pitch “doubling,” or, more generally, pitch “multiplication.” Thus, conventionally, “multiplication” of pitch by a certain factor actually involves reducing the pitch by that factor. The conventional terminology is retained herein.
Because the layer 50 of spacer material typically has a single thickness 90 (see FIGS. 1D and 1E) and because the sizes of the mask elements formed by the spacers 60 usually correspond to that thickness 90, pitch doubling typically produces mask elements of only one width. Circuits, however, generally employ features of different sizes. For example, random access memory circuits typically contain arrays of memory cells located in one part of the circuits and logic circuits located in the so-called “periphery.” In the arrays, the memory cells are typically connected by conductive lines and, in the periphery, the conductive lines typically contact interconnects in the periphery or landing pads for connecting arrays to logic. Peripheral features such as peripheral interconnects and landing pads, however, can be larger than the conductive lines in the array. In addition, electrical devices in the periphery, including peripheral transistors, can be larger than the electrical devices in the array. Moreover, even if peripheral features can be formed with the same pitch as features in the array, because mask patterns formed by pitch multiplication may be limited to those that are formed along the sidewalls of patterned photoresist, it is challenging to employ pitch multiplication while achieving the flexibility, e.g., geometric flexibility, required to define some features.
Accordingly, there is a need for methods of forming features of different sizes, especially where some features are formed below the minimum pitch of a photolithographic technique, and especially in conjunction with pitch multiplication.